diff --git a/platform/mbed_retarget.cpp b/platform/mbed_retarget.cpp
index 13d4107207..0fdf8af275 100644
--- a/platform/mbed_retarget.cpp
+++ b/platform/mbed_retarget.cpp
@@ -1241,15 +1241,8 @@ extern "C" WEAK void __cxa_pure_virtual(void)
 // SP.  This make it compatible with RTX RTOS thread stacks.
 #if defined(TOOLCHAIN_GCC_ARM)
 
-#if !defined(HEAP_START)
-/* Defined by linker script */
 extern "C" uint32_t         __end__;
 extern "C" uint32_t         __HeapLimit;
-#define HEAP_START          __end__
-#define HEAP_LIMIT          __HeapLimit
-#else
-#define HEAP_LIMIT          ((uint32_t)(HEAP_START  + HEAP_SIZE))
-#endif
 
 // Turn off the errno macro and use actual global variable instead.
 #undef errno
@@ -1258,12 +1251,12 @@ extern "C" int errno;
 // Weak attribute allows user to override, e.g. to use external RAM for dynamic memory.
 extern "C" WEAK caddr_t _sbrk(int incr)
 {
-    static  unsigned char *heap = (unsigned char *) &HEAP_START;
+    static  unsigned char *heap = (unsigned char *) &__end__;
     unsigned char *prev_heap = heap;
     unsigned char *new_heap = heap + incr;
 
     /* __HeapLimit is end of heap section */
-    if (new_heap >= (unsigned char *) &HEAP_LIMIT) {
+    if (new_heap >= (unsigned char *) &__HeapLimit) {
         errno = ENOMEM;
         return (caddr_t) -1;
     }
diff --git a/rtos/TARGET_CORTEX/TOOLCHAIN_GCC_ARM/mbed_boot_gcc_arm.c b/rtos/TARGET_CORTEX/TOOLCHAIN_GCC_ARM/mbed_boot_gcc_arm.c
index 14b1412856..5d7055062d 100644
--- a/rtos/TARGET_CORTEX/TOOLCHAIN_GCC_ARM/mbed_boot_gcc_arm.c
+++ b/rtos/TARGET_CORTEX/TOOLCHAIN_GCC_ARM/mbed_boot_gcc_arm.c
@@ -29,21 +29,10 @@ static osMutexId_t               env_mutex_id;
 static mbed_rtos_storage_mutex_t env_mutex_obj;
 static osMutexAttr_t             env_mutex_attr;
 
-#if !defined(ISR_STACK_SIZE)
-extern uint32_t               __StackLimit;
-extern uint32_t               __StackTop;
-#define ISR_STACK_START       __StackLimit
-#define ISR_STACK_SIZE        ((uint32_t)((uint32_t)__StackTop - (uint32_t)__StackLimit))
-#endif
-
-#if !defined(HEAP_START)
-/* Defined by linker script */
+extern uint32_t             __StackLimit;
+extern uint32_t             __StackTop;
 extern uint32_t             __end__;
 extern uint32_t             __HeapLimit;
-#define HEAP_START          __end__
-#define HEAP_LIMIT          __HeapLimit
-#define HEAP_SIZE           ((uint32_t)((uint32_t)HEAP_LIMIT - (uint32_t)HEAP_START))
-#endif
 
 extern void __libc_init_array(void);
 
@@ -54,10 +43,10 @@ extern void __libc_init_array(void);
  */
 void software_init_hook(void)
 {
-    mbed_stack_isr_start = (unsigned char *)ISR_STACK_START;
-    mbed_stack_isr_size = (uint32_t) ISR_STACK_SIZE;
-    mbed_heap_start = (unsigned char *) HEAP_START;
-    mbed_heap_size = (uint32_t) HEAP_SIZE;
+    mbed_stack_isr_start = (unsigned char *) &__StackLimit;
+    mbed_stack_isr_size = ((uint32_t)((uint32_t) &__StackTop - (uint32_t) &__StackLimit));
+    mbed_heap_start = (unsigned char *) &__end__;
+    mbed_heap_size = ((uint32_t)((uint32_t) &__HeapLimit - (uint32_t) &__end__));
 
     mbed_init();
     mbed_rtos_start();